From U.S. Pat. No. 4,239,697 and the corresponding German Open Application DE-OS No. 27 53 928, it is known to reduce the reactivity and shutdown of a nuclear reactor by coating free surfaces of graphitic bodies containing fissionable fuel materials with a neutron-absorbing substance.
Nuclear reactor installations generally comprise a number of control systems for regulating the reactivity of the reactor core and shutting down the chain reaction thereof. It is important that such shutdown systems be of such nature that they allow reactivation of the core in the event an emergency situation has been alleviated.
In the aforementioned U.S. patent and the corresponding German application, there is described a gas-cooled nuclear reactor having graphitic fuel elements and in which the nuclear reaction is quenched by depositing a gadolinium-containing substance on the surfaces of the fuel elements by introducing that substance into the circulating cooling gases of the primary coolant contacting the fuel elements. The more gadolinium which is deposited upon the graphite surfaces of the fuel elements, the greater will be the absorption of thermal neutrons and the greater the reduction in reactivity.
Since gadolinium in its natural isotropic mixture has the greatest neutron absorption cross-section for thermal neutrons of all naturally occurring elements, it suffices to deposit a comparatively small amount of gadolinium upon the fuel element.
If it is desired to restart the reactor, the gadolinium may be desorbed by increasing the temperature of the reactor core and flushing it with gadolinium-free gas, by scrubbing or by a nuclear decomposition. Replacement of the gadolinium-cooled fuel elements by fresh fuel elements can also restore the reactivity.
According to this earlier patent, the gadolinium compounds which are used can be applied as sols or solutions or in a gaseous state. Preferably an aqueous gadolinium acetate is used on the substance if applied as a gadolinium compound in a gaseous form, e.g. as gadolinium aluminum isopropoxide --Gd(Alc.sub.3 H.sub.7 O).sub.4).sub.3 -- or gadolinium tricyclopentadienyl --Gd(C.sub.5 H.sub.5).sub.3, the latter sublimating from the solid phase since it has no liquid phase.
In their earlier system the substances were introduced through a bypass to the cooling gas circulation and, naturally, this poses no problem when the cooling gas circulation is intact.
However, since a frequent cause of breakdown and the need for shutdown of the reactor is failure of the primary cooling system, the reliable introduction of the gadolinium compounds could not be ensured. Consequently, it is necessary to provide additional piping for the reactor to enable the introduction of the reactivity-reducing compounds.